Pez Cycling News has a very interesting series of three articles on hydration--particularly apropos, given our current 95-degree days:
Make no mistake about it, short of sickness or injury, NOTHING will affect your daily performance more than your hydration status. Study after study has demonstrated that even a very slight amount of fluid loss (1% body weight) results in significantly decreased performance in both endurance and power sports. In fact, my own Ph.D. research (2) found that proper hydration prior to exercise (i.e., making sure you’re not dehydrated to begin with) is more important than either fitness or heat acclimation in determining your tolerance to exercise in the heat.